(1) Field of the Invention
This invention relates to impulse-action heat-sealers.
Meant by impulse-action heat-sealers are such apparatuses for manufacturing sealed articles, as bags and the like, by pressingly keeping the portions to be sealed, while effecting the sealing by introducing electric current for a short period of time to heater elements of Nichrome tape or the like, thus to instantaneously heat up the heaters to the sealing temperature, and then releasing the pressing abutment after a certain cooling period of time.
(2) Description of the Prior Art
Each of such heat-sealers as mentioned above is equipped with a pressing-sealer lever adapted to come into pressing abutment against and apart freely off from a sealer surface provided in a fixed stationary location on the part of the sealer body proper, and used as the power source of this lever is the manual or motor rotational power.
Intended to improve by this invention are the heat-sealers of the motor-driven type. In the case of such motor-driven type, it is apt to incur structure-complification and cost-raising, on account that there are needed a speed-reducer and a mechanism for transmission to the pressing-sealer lever, and also that there are effected quite frequent starting-up and halting-down operations. Besides, there is the fear of causing serious trauma or injury if fingers of the sealing work operator should by any chance inadvertently be interposed in between the sealer surface and the pressing-sealer lever as they are approaching to each other, in view that the driving force causing the displacement of the pressing-sealer lever must anyhow approximately be around the press sealing force itself as required, and if a safety mechanism is provided as measures to counter same, such will then incur further complication of the structure.
In view hereof, this invention intends to solve the problem of such prior art, making use of the fact that the attraction force of a direct-current electromagnet varies sharply responsive to various positioning of the iron core, but adoption of such direct-current electromagnet incurs on the other hand the problem as now described in detail hereunder.
Referring now to FIGS. 1a, 1b, attraction force Ft of a direct-current solenoid-type electromagnet equipped with a yoke iron shows quite wide variation in strength. Thus, in the practical range of l.sub.a =l.sub.o .about.0 then ##EQU1## where I: Exciting current [A],
N: Turn number of the coil, PA1 S: Iron core sectional area [cm.sup.2 ], PA1 l.sub.a : Stroke of the iron core, PA1 L: Length of the coil (or distance between magnetic poles of fixed members).
It means thus if and when the thickness T of a finger is approximately around that as illustrated, then even if such finger should inadvertently be interposed in between the sealer surface and the pressing-sealer lever, the attraction force at such time (finger compression force) Fo would be kept small.
However, considered hereinabove is the force to be exerted under supposition that the pressing sealer lever is in standstill, while in the actual sealing operation the pressing sealer lever undergoes movement and has considerably large moving inertia at the time of inadvertently coming into abutment against the finger. Since such total inertia is then transmitted to the finger, there accordingly still exists the danger of causing injury.
On the other hand, in the case same normally operates without any such interposition of fingers, then the pressing-sealer lever has its inertia or kinetic energy increased in an accelerative manner, and since same ultimately comes to an instantaneous halting upon shocking impingement against the sealer surface (sealed article), shock sound at such time as thus generates the noise is quite large. Besides, the shocking impingement force is therefore quite large as well, and there accordingly is apt to occur the breaking damage, fatigue or the like of the pressing-sealer lever and the sealer surface.
The problems mentioned just above may as a matter of course be dissolved if the exciting current is reduced down, but such is prone to cause defective sealing on account of insufficient sealing pressure.
By the way, the direct-current electromagnet has the advantages in comparison with the alternating-current electromagnet, on account that the magnetic flux is retained without alternation, such as
1 There are no hysteresis loss and eddy current loss, thus to make it possible to use blockwise iron core of pure iron, malleable steel and the like, which are simple and easy to work on and are strong mechanically.
2 There are no fluctuations in the attraction force, thus there are no humming noises as generated thereby.
3 The exciting current is defined by pure resistance of the coil (there being no reactance factor, contrary to the alternating current) and is thus stably constant throughout the entire operation.